It is known in the art that there is a pigment ink for inkjet recording, in which a pigment such as an organic pigment or carbon black is formed into fine particles using a polymer dispersing agent or surfactant, and dispersed in water.
Also, it has been known that a resin emulsion is added to an ink, in which a pigment is dispersed using a surfactant, for improving abrasion resistance, water resistance and storage stability (see PTL 1).
However, as viscosity of an ink increases when a resin emulsion is added to the ink, it is necessary to reduce pigment concentration, which would jeopardize image density and color saturation of images to be obtained, or to reduce the amount of wetting agent, which would jeopardize jetting stability.
As another example of a dispersion system of an ink, there has been known a surface-modified pigment ink that can be stably dispersed without using a dispersing agent (e.g. a surfactant). However, printed images obtained with this surface-modified pigment ink have significantly poor abrasion resistance and water resistance.
As another example of a dispersion system of an ink, there has been proposed an inkjet ink using a microcapsule or emulsion type dispersion liquid in which pigment particles are coated with a resin (see PTL 2 etc.). However, there is a problem in this system such that the production cost is high because of its long production procedure.
As seen above, various improvements have been made in the art. However, at the time when images are printed on paper having a high penetration speed (e.g. plain paper), penetration of ink into the inside or back side of the plain paper resulted in an image having significantly a low image density and color saturation compared to images output by offset printing, screen printing, or electrophotography.
To overcome this problem, there have been proposed a recording medium (e.g. a plain paper) a surface of which has been coated with a material for fixing a coloring material thereon, and a recording medium a surface of which has been coated with a white pigment or water-soluble polymer. However, these recording media are expensive as special processing is required for the coating, and thus have not been commonly used.
To correspond to use of a common recording medium such as a plain paper, there has been proposed an inkjet recording method capable of forming an image having a high image density and color saturation on the plain paper, in which a pretreatment liquid which aggregates a coloring material is applied to a recording medium in advance, and an ink is then jetted on the area where the pretreatment liquid has been applied.
A type of a coagulating agent contained in the pretreatment liquid is selected depending on a polarity of a coloring agent, or dispersion system.
For example, in the case where a coloring agent is dispersed with assistance of an anionic surfactant or polymer dispersing agent, cationic particles or polyvalent metal salts cause strong aggregations. In this case, the speed of the aggregation reaction is fast, and thus ink droplets are fixed before they penetrate and spread on a surface of a recording medium. Therefore, a dot diameter of an ink will not increase, which efficiently suppresses bleeding between colors (color bleeding), but image density or color saturation of an image may be low. Moreover, the pigment component of the coloring agent remains only on the surface of the recording medium without penetrating into the recording medium, and thus abrasion resistance may be significantly deteriorated.
In the case where a coloring material is dispersed with assistance of a nonionic surfactant, a coagulating agent providing efficient aggregations cannot be found, and only an aggregation effect from a resin emulsion added for improving dispersion stability and fixing ability is expected. Therefore, although image density, color bleeding, and abrasion resistance are improved, sufficient effects thereof cannot be attained.
In the case where a surface-modified pigment is used as a coloring agent, polyvalent metal salts cause strong aggregations, though the effects thereof are varied depending on the type of hydrophilic groups. In this case, much like the case where the coloring material is dispersed by the anionic surfactant or polymer dispersing agent, the speed of the aggregation reaction is fast, and thus the color bleeding is suitably prevented, but color density or color saturation of an image may be low due to excessively controlled dots, providing insufficient abrasion resistance.
In the case where a resin-coated pigment is used as a coloring material, organic acids or polyvalent metal salts cause aggregations, though the effect thereof varies depending on the type of hydrophilic groups. In this case, the speed of the aggregation reaction is relatively slow, and a dot diameter of an ink is large. Therefore, the resulting image has high image density and color saturation, and provides relatively excellent abrasion resistance. However, the color bleeding cannot be efficiently prevented.
As have been mentioned above, there are advantages and disadvantages in the combination of the dispersion system and pretreatment liquid for use. Therefore, there has been proposed an inkjet recording method using an ink in which these pigment dispersion systems are mixed, and a pretreatment liquid (see PTL 3). PTL 4 and the like propose a combination of a mixed ink having dispersed particles using a surface-modified dispersed particles and polymer dispersing agent, and a pretreatment liquid. PTL 5 discloses a combination of a mixed ink having dispersed particles subjected to different surface modifications, and a pretreatment liquid containing a polyvalent metal salt. However, the speed of the aggregation reaction between the surface-modified dispersed particles or dispersed particles using the polymer dispersing agent, and the polyvalent metal salt is extremely fast, and thus image density and color saturation of an image may be low because printed dots will not spread, or abrasion resistance may be low as the pigment component of the coloring material remains on a surface of a recording medium. Therefore, these problems have not yet been solved.
As have been mentioned above, there are various types of dispersion systems of a pigment and pretreatment liquids containing a coagulating agent, and various combinations thereof have been proposed. However, it is a current situation that an ink, or a combination of an ink and pretreatment liquid that produces images satisfying excellent image density, color saturation, prevention of color bleeding, and abrasion resistance at the same time, is not available.